J. Electroanal. Chem., 83 (1977) 117--129 117 © Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands

INHIBITION OF THE ELECTROOXIDATION OF Cr(II) BY ADSORBED Cr(NCS) 3

DALE HALL *, A.L. FELIU ** and D.A. AIKENS

Department of Chemistry, Rensselaer Polytechnic Institute, Troy, N.Y. 12181 (U.S.A.)

(Received 17th March 1976; in revised form 21st October 1976)

ABSTRACT

The electrooxidation of Cr(II) in the presence of 1--10 mM thiocyanate is inhibited over a wide potential range by adsorbed Cr(NCS)3. Blocking of the electrode and a repulsive diffuse double layer effect appear to be the major inhibiting mechanisms. The inhibition is removed at anodic potentials probably by faradaic removal of Cr(NCS)3 and at cathodic potentials by reduction of Cr(NCS) 3. Under the latter conditions an anodic "pseudo-prewave" is observed. The electroreduction of Cr(III)-thiocyanate complexes is discussed as it relates to the con- trolled potential electrooxidation of Cr(II) in the presence of thiocyanate and as it influences the electrooxidation of Cr(II).

INTRODUCTION

The electrooxidation of Cr(II) in the presence of thiocyanate was first studied by Pecsok and Lingane [1], who observed that the anodic polarographic wave occurs at much less anodic potentials in thiocyanate than in other common elec- trolytes. As an example, the half-wave potential is 0.37 V less anodic in 0.05 M thiocyanate than in 0.1 M chloride. On the basis of the formation constants of Cr(II)- and Cr(III)-thiocyanate complexes [2], no more than 0.10 V of this shift can be attributed to the influence of thiocyanate on the formal potential of the Cr(III)/Cr(II) couple. Hence, the anodic shift of the half-wave potential in thio- cyanate media must be attributed primarily to enhancement of the rate of oxida- tion of Cr(II) by thiocyanate.

The anodic polarography of Cr(II) in thiocyanate solution is also characterized by current suppression over much of the limiting current range. The suppression of the limiting current was observed by Pecsok and Lingane [1| , who noted that the current-time curves were erratic in the range of current suppression, but did not investigate the nature of the current suppression. In the present paper, the current suppression is shown to be associated with adsorption of Cr(NCS)s. The electroreduction of Cr(NCS)a is discussed as it affects the current suppression and the product distribution obtained by controlled potential oxidation of Cr(II) in the presence of thiocyanate.

* Present address: Exxon Research and Engineering Company, Linden, N.J. 07036, U.S.A. ** NSF Undergraduate Research Participant 1974; present address: Department of Chemistry,

Harvard University, Cambridge, Mass. 02138, U.S.A.

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EXPERIMENTAL

Solutions of Cr(II) were prepared by electroreduction of chromic perchlorate in 1 M perchloric acid at a stirred mercury cathode at a potential of --0.90 V vs. SCE. Reduct ion was at least 98% complete as indicated by the Cr(III) content before reduction and the Cr(II) content after reduction. The total Cr(III) con- tent was determined spectrophotometrically as chromate [4] and the Cr(II) con- tent was determined volumetrically by reaction of an aliquot with deoxygenated Fe(III) followed by titration in 1 M phosphoric acid with dichromate to the di- phenylamine sulfonate endpoint. Solutions of bis- and tris-thiocyanate com- plexes of Cr(III) were prepared chemically by reaction of Cr(III) perchlorate with sodium thiocyanate and separated by ion exchange as described by Barclay et al. [3]. Once separated, the Cr(III) complexes were stored in an ice bath and used within four hours. It was determined that no more than 2% of any Cr(III)- thiocyanate complexes is hydrolyzed during the timespan of the following elec- trochemical measurements.

Deaerated reagents were added to the cell through a septum using a hypoder- mic syringe. Nitrogen for deaeration was passed through two chromous chloride scrubbers before entering the cell and through a trap after passing through the cell. With these precautions, the Cr(II) limiting current decreased by less than 1% per hour. A conventional 3-electrode polarograph was used with an isolated Ag/ AgC1 reference electrode (0.1 M NaC1) which has a potential of +0.041 V with respect to the SCE. Experiments were performed at 24 -+ I°C.

RESULTS AND DISCUSSION

Anodic polarography of Cr(II) in the presence of thiocyanate

Polarograms of 3.6 mM Cr(II) in 1 M HC104 and in 1 M HC104 containing thiocyanate at concentrations ranging from 0.6 mM to 41 mM are given in Figs. l a and b. In contrast to the relatively simple current-potential curves observed in the ligand catalyzed deposition of nickel [5] or of indium [6], the morpholo- gy of the anodic Cr(II) current-potential curves in Fig. 1 is indeed complex, and this suggests that the electrooxidation of Cr(II) in the presence of thiocyanate is more complex than these ligand catalyzed electrodepositions. Three major fea- tures are evident in the current-potential curves: a prewave, a zone of current suppression, and an abrupt increase of the current to the diffusion-limited level. The relative prominence of each of these features depends in a systematic fashion on the thiocyanate concentration as described below.

In the absence of thiocyanate, the electrooxidation of Cr(II) is characterized by the drawn-out, irreversible current-potential curve A shown in Fig. 1A. Addi- tion of small concentrations of thiocyanate causes development of a prewave at the foot of tile main wave as illustrated by curve B (0.6 mM SCN-) and curve C (1.2 mM SCN-), and this prewave is similar to those observed in the ligand cat- alyzed deposition of nickel [5] and indium [6].

At thiocyanate concentrations of 1.6 mM (curve D) and above, the prewave develops a current peak, which increases in magnitude as the thiocyanate con- centration is raised. Individual current-time curves are irregular in the vicinity of

119

14

12

I0

,~ 8

4

2

0 I

0.2

25

1 5

I 0

I

0.0

Q

E

-0.2 -0.4 -0.6 -0.8 E / V vs SCE

0

0.2

I I I I I I I

0.0 - 0 . 2 - 0 . 4 - 0 . 6 E / V vs SCE

-0.8

Figs. la and b. Anodic current-potential curves for 3.6 mM Cr(II) in 1 M HC104 containing various concentrations of SCN-. Curve: A B C D E F G H [SCN--]/mM: 0.0 0.6 1.2 1.6 2.9 3.5 7.1 41

the current peak, especially at the higher thiocyanate concentrations, indicating convective stirring. As the thiocyanate concentration is increased, the prewave shifts continuously toward more negative potentials, as indicated by the half- peak potential, which shifts from --0.55 V vs. SCE at a thiocyanate concentra- tion of 0.6 mM {curve B) to --0.65 V vs. SCE at a thiocyanate concentration of 41 mM (curve H). The prewave is essentially fully developed at a thiocyanate concentration of 41 mM, and the peak current is somewhat larger than the diffu- sion-limited value because of convection.

The p~ewave is, in general, followed at more anodic potentials by a zone of current suppression, which first becomes apparent at a thiocyanate concentra- tion of 1.2 mM (curve C), and in which the current increases as the thiocyanate concentration is raised. The onset of the current suppression occurs over a narrow range of thiocyanate concentrations, as is indicated by comparison of curve C (1.2 mM S C N - ) and curve D (1.6 mM SCN-) . Current suppression is just detect-

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able in curve C, whereas it is essentially fully developed in curve D. In curve D, the current is suppressed to approximately 25% of the diffusion-limited value and remains so until the potential is increased to --0.05 V vs. SCE, at which point it rises sharply to the diffusion-limited value. When the thiocyanate con- centration is increased to 2.9 mM {curve E), the current-potential curve is super- ficially similar to curve D, but it differs in two significant respects. First, the de- gree of current suppression, as defined by the ratio of the suppressed current to the diffusion-limited current, is somewhat less, and second, the current remains suppressed until the potential is increased to +0.15 vs. SCE, a value 0.20 V more anodic than in curve D. Thus, increasing the thiocyanate concentration diminishes the severity of the current suppression as defined by the magnitude of the suppressed current, and also shifts the anodic limit of the current suppression zone to a significantly more anodic potential. Both effects become more pro- nounced as the thiocyanate concentration is increased, and at thiocyanate con- centrations of approximately 40 mM, the current is nearly the diffusion-limited value over the accessible potential range. Curves F (3.5 mM SCN-) and G (7.1 mM SCN-) illustrate the continued systematic anodic shift of the anodic limit of the current suppression zone and the lessening of the severity of current sup- pression. Curve H (41 mM SCN-) is truncated at 0.00 V vs. SCE, because the current rises abruptly to a very high level of slightly more anodic potentials. This factor prevents observation of the anodic limit of the current suppression zone, but the degree of current depression is slight at this thiocyanate concentration.

The prewave and the current suppression indicate that the electrooxidation of Cr(II) is subject to both enhancement and inhibition in the presence of thio- cyanate. The enhancement is attributed to ligand bridging. It has been shown that halides accelerate the electrooxidation of Cr(II) by a ligand bridge mecha- nism [7--9], and it has also been shown that the electrooxidation of Cr(II) in the presence of thiocyanate involves a multiple ligand bridge mechanism [3]. The operation of the multiple ligand bridge mechanism is probably the major reason that thiocyanate accelerates the electrooxidation much more effectively than do the halides.

The morphology of the current-potential curves which exhibit strong current suppression suggests that current suppression arises from adsorption of a reactant or product. Curves D, E and F of Fig. 1, which demonstrate severe current suppression, correspond very closely to the morphology associated with such adsorption as summarized by Laviron [10]. This process, termed autoinhibi- tion by Laviron [11 ], is of ten encountered in organic polarography, and although autoinhibition is less common in inorganic polarography, it is by no means un- known. Two well documented examples are the inhibition of the reduction of Cr(VI} by Cr(OH)~ [12] and the inhibition of the reduction of As(III) [13--15] by As(0). Although the inhibiting films in these examples are solids, observation of inhibition requires only that the film be strongly absorbed. Additional evi- dence that adsorption causes the current suppression is the narrow range of thio- cyanate concentrations (0.6--1.6 mM) over which the suppression develops. The abruptness of the onset of current suppression in terms of thiocyanate concentra- tion is similar to the strong concentration dependence found by Gross and Murray [16], who showed that adsorbed lead halides inhibit the electroreduction of Hg(II) in halide supporting electrolytes.

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Anson et al. [3,17] have shown ~hat Cr(NCS)2 + and Cr(NCS)3 are major prod- ucts of the electrooxidation of 10 mM Cr(II) in the presence of 20 mM thiocya- nate, and that both Cr(NCS)2 ÷ and Cr(NCS)3 are strongly adsorbed at mercury electrodes at potentials anodic of --0.55 V vs. SCE. They also showed that little CrNCS 2÷ is formed under these conditions and that CrNCS 2÷ is not appreciably adsorbed in this potential range. Hence, among the Cr(III) products, both Cr- (NCS)2 ÷ and Cr(NCS)3 are potential inhibitors of the electrooxidation of Cr(II). No data are available concerning the adsorption of Cr(II)-thiocyanate species, so that no predictions can be made concerning their effectiveness as inhibitors. Analysis of the influence of Cr(NCS)2 ÷ and Cr(NCS)3 on the electrooxidation of Cr(II) is complicated by the electroreduction of these Cr(III) species over part of the potential range of interest, which prevents their accumulation at the elec- trode surface. As a result, the electroreduction properties of the various Cr(III)- thiocyanate species must be considered to achieve an understanding of the elec- trooxidation of Cr(II) in the presence of thiocyanate.

Drop time studies of Cr(II)-thiocyanate solutions

Drop time studies of Cr(II) in the presence of low concentrations of thiocya- nate confirm that the current suppression is indeed associated with strong ad- sorption of one or more species. Figures 2a and b show polarographic and drop- time curves for solutions of 4.25 mM Cr(II) in 1 M HC104, with added thiocya- nate ion at concentrations ranging from 1.5 mM to 2.4 mM, the concentration range corresponding to the onset of current suppression. These results were ob-

12

I a

8

4

7.2

5.6 b

0,1 0.0 -0.1 -0 .2 - 0 3 - 0 . 4 - 0 . 5 lOl 6 "0.7

E / V VS SCE

Fig. 2. Anod ic cu r ren t -po ten t i a l curves (a) and d rop- t ime po ten t i a l curves (b) or Cr(II) in 1 M HC10 4 con ta in ing various concen t r a t i ons of S C N - . Curve : A B C D [SCN--] /mM: 0.0 1.5 2.0 2.4

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tained with a different capillary than was used for the data in Fig. 1, and there- fore the currents are different.

The current-potential curves shown in Fig. 2a exhibit features generally simi- lar to those curves in Fig. 1 which represent comparable thiocyanate concentra- tions. The corresponding drop-time potential curves given in Fig. 2b indicate that systematic depression of the surface tension of mercury is associated with the electrode reaction and that it becomes more pronounced as the thiocyanate concentration is increased. Curve A corresponds to 1 M HC1Oa, and curves B, C, and D correspond to 4.25 mM Cr(II) in 1 M HClO4 with added thiocyanate con- centrations of 1.5, 2.0, and 2.4 mM, respectively. At potentials more cathodic than --0.70 V vs. SCE, all four drop-time curves coincide, indicating that the presence of free thiocyanate does not significantly decrease the mercury-solu- tion interfacial tension in this potential range. As the potential is made more anodic, the drop-time curves for the solutions containing thiocyanate (B, C, and D) progressively diverge from the drop-time curve for the thiocyanate-free solu- tion (A). The point-by-point nature of the drop-time potential curves masks the fine structure except for those portions where the points are taken at narrow po- tential intervals, but the general nature of the curves is clear. At a sufficiently anodic potential, the value of which is dependent on the thiocyanate concentra- tion, the drop-time abruptly increases to a value which is only slightly less than that observed for the thiocyanate-free solution. At potentials anodic of this tran- sition, the drop-time curves of the solution containing thiocyanate remain slight- ly below the drop-time curve for the thiocyanate-free solution.

The critical result is that at each thiocyanate concentration studied, the abrupt increase in the drop-time curve in Fig. 2b occurs at precisely the same po- tential as the termination of the current suppression in Fig. 2a. Depression of the drop-time of a dropping mercury electrode at fixed potential and in the ab- sence of a change in the electrode composit ion is generally understood to be an indication of adsorption at the mercury surface. The precise correspondence of the discontinuity of each polarogram in Fig. 2a with the discontinuity of the corresponding drop-time potential curve in Fig. 2b indicates that the current suppression is caused by adsorption of one or more thiocyanate complexes of Cr(II) or Cr(III), but these results alone do not indicate which species suppresses the current nor do they indicate the mechanism which terminates adsorption. The strong adsorption of Cr(NCS)2 ÷ and Cr(NCS)3 suggests that either or both of these Cr(III) species are involved, and the effects of these species on the elec- t rooxidat ion of Cr(II) are reported below.

Inhibition of Cr(II) electrooxidation by Cr(III)-thiocyanate complexes

The influence of Cr(NCS)2 ÷ and of Cr(NCS)3 on the electrooxidation of Cr(II) in the presence of ca. 1 mM thiocyanate was studied using chemically pre- pared complexes in 0.2 M HC104 to avoid acid-induced degradation of the Cr- (III)-thiocyanate complexes. Because this electrolyte is much more dilute than the 1 M HC104 which was used to obtain the results in Figs. I and 2, the present results cannot be compared directly with those in Figs. 1 and 2.

The effects of Cr(NCS)2 + and Cr(NCS)3 on the electrooxidation of Cr(II) indi- cate that the current discontinuity in Figs. 1 and 2a is associated with Cr(NCS)s

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but not with Cr(NCS)2 +. This conclusion follows directly from the influence of added Cr(NCS)2 ÷ and Cr(NCS)a on the potential of the current discontinuity. The drop-time studies in Fig. 2b show that the inhibitor is strongly absorbed at potentials cathodic of the current discontinuity, and that it is not adsorbed at potentials anodic of the current discontinuity. Increasing the concentration of the species associated with the current discontinuity shifts both the anodic limit of adsorption and current discontinuity anodically. Thus the influence of the concentration of the Cr(III)-thiocyanate complexes on the potential of the cur- rent discontinuity provides a basis for determining whether these complexes are associated with the current discontinuity.

The effect of submillimolar concentrations of Cr(NCS)2 + on the anodic polarog- raphy of 4.2 n~ l Cr(II) in 0.2 M HC104 in the presence of 1.1 mM thiocyanate is shown in Fig. 3. The polarograms were recorded after addition of successive aliquots of 8.6 mM Cr(NCS)2 + stock solution, and thus they reflect the effect of dilution, which reaches 5% at the maximum Cr(NCS)2 ÷ concentration of 0.41 mM. The suppression of the Cr(II) anodic current between +0.04 V and +0.20 V vs. SCE cannot be attributed entirely to dilution, and it may represent electro- static repulsion of Cr(II) by adsorbed Cr(NCS)2 ÷. The critical result is the failure of added Cr(NCS)2 + to shift the current discontinuity anodically, which demon- strates that the current suppression is not caused by adsorption of Cr(NCS)2 + . The current discontinuity actually undergoes a slight cathodic shift as the con- centration of Ce(NCS)2 + is increased, and this is attributed to concurrent dilu- tion of the thiocyanate concentration. Figures 1 and 2a confirm that the current discontinuity shifts cathodically as the thiocyanate concentration is decreased.

The possibility that the added Cr(NCS)2 + fails to shift the current discontinu- ity anodically in these experiments because the concentration of Cr(NCS)2 ÷ added was insignificant relative to that formed in the electrode reaction can be

12.5

I0.0 ~ j

7.5

• - A 5.0

2.5

0.0 0 2 0.0 -0.2 -04 -06

E/V vs SCE

Fig. 3. Ef fec t of added Cr(NCS)2 + on the anodic cu r ren t -po ten t i a l curve of Cr(II) in 0.2 M HCI04, ca. 1.1 mM SCN--. Curve: A B C Added [Cr (NCS)2÷] /mM: 0.00 0.25 0.41 [SCN-- ] /mM: 1.12 1.08 1.05

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ruled out. The maximum concentration of Cr(NCS)2 ÷ which could be formed electrochemically is essentially equal to one half the thiocyanate concentrat ion or 0.55 mM, and the actual concentration is probably less than this. Hence, the added concentration of Cr(NCS)2 ÷ (0.41 mM) is comparable to the concentra- tion which might reasonably be formed electrochemically.

The effect of submillimolar concentrations of Cr(NCS)3 on the anodic polarog- raphy of 4.2 mM Cr(II) in 0.2 M HC104 in the presence of 1.2 mM thiocyanate is shown in Fig. 4. Except for the nature of the added Cr(III)-thiocyanate com- plex, the conditions for this experiment are identical to those for the study of Cr(NCS)2 ÷. The Cr(NCS)3 was added as a 3 mM stock solution, and the decrease in the Cr(II) limiting current is attributable to dilution of the Cr(II) concentra- tion. The critical result is the progressive anodic shift of the current discontinu- ity as the Cr(NCS)~ concentration is increased, which demonstrates that the cur- rent suppression is associated with adsorption of Cr(NCS)3.

The influence of Cr(III)-thiocyanate complexes higher than Cr(NCS)3 on the electrooxidation of Cr(II) is not known. Consideration of the thiocyanate con- centration, the Cr(III) product distributions reported by Barclay et al. [ 3], and the th iocyana te concentrations in the present study indicate that significantly less than 5% of the Cr(III) product is present as higher complexes than Cr(NCS)8 in the experiments in which the electrooxidation is strongly inhibited. Attempts to detect higher complexes than Cr(NCS) 3 among the products of the electroxi- dation of Cr(II) under these conditions were unsuccessful.

Both Cr(NCS)2 + and Cr(NCS)3 lower the Cr(II) prewave to a degree that ex- ceeds the effect of dilution, a result which is attributed to the adsorptive proper- ties and polarographic behavior of these complexes. In the potential range of in- terest, --0.40 V to --0.70 V, Cr(NCS)2 ÷ is probably adsorbed to a significant ex- tent thus lowering the Cr(II) anodic current by electrostatic repulsion of Cr(II).

12.5

I0.0

< 7.5

5.0

2.5

0.0

c

0.2 0.0 -0 .2 - 0 . 4 -0 .6 E / V vs S C E

Fig. 4. Ef fec t of added Cr(NCS)3 on the anodic current -potent ia l curve of Cr(II) in 0.2 M HClO4, ca. 1.2 mM SCN--. Curve: A B C D Added [Cr(NCS)3] /mM: 0.00 0.06 0.18 0.28 [SCN-- ] /mM: 1.21 1.19 1.14 1.09

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The equilibrium adsorption of Cr(NCS)2 + has been measured by Anson et al. [3,17] at potentials as cathodic as --0.30 V, but the reduction of the complex prevented study at more cathodic potentials. The reduction of Cr(NCS)2 ÷ is not sufficient, however, to lower the Cr(NCS)2 ÷ concentration significantly in the potential range of the Cr(II) prewave, because the foot of the Cr(NCS)2 + polaro- graphic wave does not occur until the potential reaches --0.50 V and the Cr- (NCS)2 ÷ reduction current rises only slowly with increasing potential. A signifi- cant portion of the Cr(NCS)2 ÷ thus remains to impede the oxidation of Cr(II). The reduction of Cr(NCS)3 requires significantly less cathodic potentials than the reduction of Cr(NCS)2 ÷, and the foot of the Cr(NCS)3 cathodic wave occurs at approximately --0.40 V. As a result the reduction of Cr(NCS)3 contributes a significant cathodic current in the potential range of the Cr(II) prewave, thereby lowering the magnitude of the anodic prewave.

The prewave at the foot of the anodic Cr(II) wave indicates that the current suppression is relieved at cathodic potentials because Cr(NCS)3 does not accumu- late at the electrode surface. Over a range of thiocyanate concentrations, the transition from prewave to current suppression in Fig. 1 occurs at approximately --0.40 V vs. SCE, which corresponds closely to the potential of the foot of the cathodic polarographic wave of Cr(NCS)3. At potentials more cathodic than - 0 . 4 0 V vs. SCE the surface concentration of Cr(NCS)3 is depleted, and the elec- trooxidation of Cr(II) is not suppressed, whereas at more anodic potentials, Cr(NCS)3 accumulates, and the electrooxidation of Cr(II) is suppressed. Laviron [ 11 ] has suggested the term "pseudo-prewave" for this situation to emphasize that it differs in an important manner from the classical prewave caused by prod- uct adsorption.

Cathodic polarography of Cr(III)-thiocyanate complexes

The electroactivity of the various Cr(III)-thiocyanate complexes can influence to a significant extent the product distribution obtained by electrooxidation of Cr(II} in the presence of thiocyanate. The cathodic current-potentials curves of Cr(NCS)2 +, Cr(NCS)2 ÷ and Cr(NCS)3 overlap to varying degrees the anodic cur- rent-potential curve of Cr(II) in the presence of moderate concentrations of thiocyanate. The extent to which these cathodic current-potential curves over- lap the anodic current-potential curve at a given potential has an important bear- ing on the distribution of Cr(III)-thiocyanate complexes which will be formed by electrooxidation of Cr(II) at the potential in question.

At potentials sufficiently anodic that none of the Cr(III)-thiocyanate com- plexes are reducible, the electrooxidation of Cr(II) proceeds in a completely irre- versible manner. Because the various Cr(III)-thiocyanate products are essentially substitution inert, even in the presence of excess Cr(II), the distribution of Cr- (III)-thiocyanate complexes is determined solely by the relative rates at which they are formed. That is, the relative amounts of the various Cr(III)-thiocyanate complexes are determined by the relative concentrations of the corresponding Cr(II) complexes at the electrode surface and the rate constants for their elec- trooxidation. Under such conditions, the distribution of Cr(III)-thiocyanate products can be explained in terms of the density of adsorbed thiocyanate, which determines the probability of forming the various Cr(II)-thiocyanate

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species on the electrode surface, as proposed by Barclay et al. [3]. At a potential sufficiently cathodic that one of the Cr(III)-thiocyanate com-

plexes is reducible, however, the ne t ratio of formation of that Cr(III)-thiocya- nate Complex is no longer determined solely by the rate constant for electrooxi- dation of the pertinent Cr(II) complex and its concentration at the electrode surface. The net rate of formation of the Cr(III)-thiocyanate complex is now the difference between the rate of its formation by electrooxidation and the rate of its disappearance by electroreduction to Cr(II) and thiocyanate. The importance of the reverse reaction depends, of course, on the relative magnitudes of the anodic and cathodic currents. As the current for electroreduction of the Cr(III) thiocyanate complex in point increases relative to the current for electrooxida- tion of Cr(II), the correlation of the concentration of this Cr(III) complex in the product mixture with the forward rate of formation becomes increasingly poorer. Hence, if a particular Cr(III)-thiocyanate species is electroactive at a given poten- tial, the concentration of this species produced by electrooxidation of Cr(II) in the presence of thiocyanate will not correlate with the density of thiocyanate adsorbed on the electrode surface or other factors which relate only to the rate of formation of the Cr(III)-thiocyanate complex.

The current-potential curve for oxidation of Cr(II) in the presence of moder- ate thiocyanate concentrations does in fact overlap the current-voltage curves for reduction of the various Cr(III)thiocyanate products, to a degree dependent on the identity of the Cr(III)-thiocyanate species. The overlap is greatest in the case of Cr(NCS)3, and is much less in the case of Cr(NCS)2 + or CrNCS 2+, as indicated by the respective half-wave potentials, which for Cr(NCS)3, Cr(NCS)2 ÷ and CrNCS 2+ are --0.56, --0.70 and --0.68 V vs. SCE. The reduction of Cr(NCS)a be- comes significant at potentials as anodic as --0.40 V vs. SCE, whereas Cr(NCS)2 + and Cr(NCS) 2÷ do not undergo significant reduction at potentials more anodic than - 0 . 5 2 V vs. SCE. Hence, the fraction of Cr(NCS)3 in the product mixture when Cr(II) is subjected to electrooxidation in the presence of thiocyanate at a potential more cathodic than --0.40 V vs. SCE must be lower than the fraction of Cr(NCS)3 in the product mixture when the electrooxidation is performed at a potential more anodic than - 0 . 4 0 V vs. SCE.

The potential dependence of the distribution of Cr(III)-thiocyanate species ob- tained by Barclay et al. [3] in controlled potential oxidation of Cr(II), can be at- tr ibuted at least partially to this effect. Whereas approximately 50% of the Cr(III) product was Cr(NCS)3 on electrooxidation at - 0 . 1 5 V vs. SCE or --0.30 V vs. SCE, approximately 5% of the Cr(III) product was Cr(NCS) 3 on electrooxi- dation at - 0 . 5 5 V vs. SCE. The percentage yields of both CrNCS 2+ and Cr(NC$)2 + were increased accordingly at the more negative potential. These changes in the Cr(III) product distribution were attr ibuted to the larger average spacing of ad- sorbed thiocyanate ions at - 0 . 5 5 V which hinders formation of the tris-Cr(II) reactant complex on the electrode surface and thus favors formation of mono- and bis-Cr(III) species. The spatial arrangement of adsorbed thiocyanate certain- ly influences the Cr(III) product distribution, but it seems unlikely that this fac- tor alone can account for the precipitous drop in the yield of Cr(NCS) 3 between - 0 . 3 0 V vs. SCE and - 0 . 5 5 V vs. SCE. A major factor leading to the low yield of Cr(NCS)3 at --0.55 V vs. SCE is that Cr(NCS)3 is electroactive at this poten- tial so that the yield of Cr(NCS)3 is reduced accordingly.

127

Mechanism of current suppression

The suppression of the electrooxidation of Cr(II) in the presence of adsorbed Cr{NCS)3 manifested by the unusual current-potential curves of Figs. 1 and 2 re- flects the competitive interactions of factors which favor the electrode reaction and factors which oppose the electrode reaction. Three such factors have been identified and their influence on the electrode reaction is described qualitatively in the following sections. The three factors are blocking of the electrode surface by adsorbed Cr(NCS)3, the diffuse double layer effect associated with the sup- porting electrolyte which opposes the electrode reaction, and a second diffuse double layer effect associated with adsorbed Cr(NCS)3 which favors the electrode reaction.

Blocking of the electrode surface by adsorbed Cr(NCS)3 appears to be a prin- cipal mechanism of current inhibition, because conditions which cause substan- tial current suppression also cause extensive surface coverage. Essentially com- plete surface coverage requires very low bulk concentrations of Cr(NCS)3, as shown by Anson and Rodgers [17], who found that complete surface coverage at --0.1 V occurs when the bulk concentration of Cr(NCS)3 is only ca. 0.1 mM. The thiocyanate concentration which corresponds to the most severe current suppression in Figs. I and 2a is ca. 1.5 mM, which in a stoichiometric sense is equivalent to a Cr(NCS)3 concentration of 0.5 mM, a concentration fivefold that required for saturation coverage. That saturation coverage does in fact result un- der these conditions is indicated by the drop-time data in curves B, C and D of Fig. 2b, which show that the drop-time at a given potential in the potential range corresponding to current suppression is essentially independent of the thiocya- nate concentration. It is very improbable that the drop-time would be indepen- dent of the thiocyanate concentration unless essentially complete coverage is at- tained at the lowest thiocyanate concentration so that all higher thiocyanate concentrations studied correspond to complete coverage.

The potential distribution in the diffuse double layer in the potential range of interest is such that it opposes the electrooxidation of Cr(II), and this double layer effect becomes increasingly important when the supporting electrolyte con- centration is reduced. The potential range of interest is positive of the point of zero charge, and standard double layer theory [18,19] predicts that in this poten- tial range ~2, the potential at the outer ttelmholtz plane, in positive. The resulting electrostatic repulsion of the positively charged Cr(II) reactant and the reduction of the effective potential for electrooxidation inhibit the electrooxidation of Cr(II), and the magnitude of the effect becomes smaller as the electrolyte concen- tration is raised. This diffuse double layer effect is similar to that observed by Gierst and Cornelissen [20] in the electrooxidation of Eu(II) in perchlorate me- dium in the same potential range. The operation of this diffuse double layer ef- fect results in greater suppression of the current when the HC104 concentration is reduced from 1 M to 0.2 M as demonstrated by comparison of polarograms corresponding to equal thiocyanate concentrations but different HC1Ot concen- trations. Curve C of Fig. 1 and curve A of Fig. 3 both correspond to thiocyanate concentration of 1.2 mM, but the data in Fig. 1 correspond to an HC104 con- centration of 1 M, whereas the data in Fig. 3 correspond to an HcLO4 concen- tration of 0.2 M. In 1 M HC104, the degree of current suppression is very small,

128

but in 0.2 M HC104, the current is strongly suppressed. The influence of the diffuse double layer effect on the electrooxidation of

Cr(II) is countered by a second double layer effect which arises from the influ- ence of adsorbed Cr(NCS)3. Adsorption of Cr(NCS)3 tends to decrease ¢2 and thus to counter the primary diffuse double layer effect. Saturation coverage of a mercury electrode with Cr(NCS)3 in 1 M NaC104 at --0.1 V, as shown by Weaver and Anson [21], increases the positive charge density of the metallic phase by 6 gC cm -2. Because the charge on the solution side of the interface must be equal in magnitude but opposite in sign to the charge on the metal, the charge density of the solution phase must become more negative by 6 pC cm -2, and the value of ~2 must decrease. Hence the electrostatic repulsion of Cr(II) diminishes and the effective potential for electrooxidation increases as a result of the adsorption of Cr(NCS)3. The effect of adsorption of Cr(NCS)3 on the potential distribution in the double layer is thus similar to the effect of anion adsorption, which Gierst and Cornelissen [20] found accelerates the electrooxi- dation of Eu(II). The favorable effect of anion adsorption on ¢2 may also be re- sponsible for the steady increase in the suppressed current with increasing thio- cyanate concentration illustrated in Figs. la and b. Whether the gradual removal of current suppression with increasing thiocyanate concentration is best attri- buted to partial displacement of adsorbed Cr(NCS)3 by thiocyanate, to a change in the distribution of Cr(II)-thiocyanate products or to some more subtle effect is not clear. Barclay et al. [3] have shown that the surface excess of Cr(NCS)3 is decreased significantly by thiocyanate concentrations as low as 5 mM. It can also be anticipated that at sufficiently high thiocyanate concentrations, competitive adsorption of thiocyanate and/or formation and adsorption of anionic Cr(III)- thiocyanate complexes will become significant.

The abrupt removal of adsorbed Cr(NCS)a from the electrode surface at posi- tive potentials, which sets the anodic limit of the current suppression zone, is striking because of its sharpness. Although the sharpness of the transition might sug- gest that Cr(NCS)3 is removed by desorption, closer analysis, based on a thermody- namic argument given by Weaver and Anson [21], indicates that desorption of Cr(NCS)a at positive potentials is improbable. Because adsorption of Cr(NCS)3 increases the positive charge density of the metal, the extent of adsorption must increase as the potential is made more anodic. Hence, desorption of adsorbed Cr(NCS)a at positive potentials appears to be impossible. Removal of adsorbed Cr(NCS)a may involve a faradaic process, such as formation of an Hg(II)-Cr- (NCS)3 adduct, as was suggested by Anson and Rodgers [17], based on the exis- tence of such adducts as demonstrated by Armor and Haim [22], who studied the formation of the Hg(II) adduct of Cr(NCS) 2+.

The abruptness of the removal of adsorbed Cr(NCS)3 from the electrode is attributed to attractive interactions between adsorbed Cr(NCS)3 molecules. Such interactions, which were discovered by Anson et al. [ 17,21], increase the relative stability of the adsorbate at higher adsorption coverage. Once the potential ex- ceeds the value necessary to disrupt the initial stable configuration correspond- ing to high coverage of Cr(NCS)3, the attractive interaction is lowered progres- sively as Cr(NCS)3 is removed. Thus, only a small increase in potential is re- quired to remove the Cr(NCS)3 entirely.

129

ACKNOWLEDGMENT

The helpful advice of Dr. F.C. Anson is gTatefully acknowledged.

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